How can I select multiple folders to convert?
Audacity only supports a single folder.
Batch converting across multiple folders is likely to require a specialist batch converting app, or writing your own script. There’s a forum topic with some information about writing batch conversion scripts here: https://forum.audacityteam.org/t/batch-scripts-with-other-programs/18178/1
If you simply want to convert formats [u]TAudioConverter[/u] can convert to/from many different formats and it can convert multiple files at once. Once you’ve set-up the output-format and output destination, it’s just one click.